The invention relates to a diode-pumped solid-state laser and, in particular, to employing such a laser to generate an ultraviolet laser beam having a TEM00 non-astigmatic spatial mode to drill vias.
U.S. Pat. Nos. 5,593,606 and 5,841,099 of Owen et al. describe techniques and advantages for employing UV laser systems to generate laser output pulses within advantageous parameters to form through-hole or blind vias through at least two different types of layers in multilayer devices. These parameters generally include nonexcimer output pulses having temporal pulse widths of shorter than 100 ns, spot areas with spot diameters of less than 100 xcexcm, and average intensities or irradiances of greater than 100 mW over the spot areas at repetition rates of greater than 200 Hz.
Lasers are described herein only by way of example to ultraviolet (UV) diode-pumped (DP) solid-state (SS) TEM00 lasers that generate a natural Gaussian irradiance profile 10 such as shown in FIG. 1, but the description is germane to almost any laser generating Gaussian output. Ablating particular materials with any laser, and particularly a UV DPSS laser, is contingent upon delivering to a workpiece a fluence or energy density (typically measured in units of J/cm2) above the ablation threshold of the target material. The laser spot of a raw Gaussian beam can be made quite small (typically on the order of 10 to 15 xcexcm at the 1/e2 diameter points) by focusing it with an objective lens. Consequently the fluence of the small focused spot easily exceeds the ablation threshold for common electronic packaging materials, particularly the copper typically used in the metallic conductor layers. Hence, the UV DPSS laser, when used in a raw, focused beam configuration, is an excellent solution for drilling vias through one or more copper layers in an electronic packaging workpiece. Since the focused spot is typically smaller than the desired size of the via, the focused spot is moved in a spiral, concentric circular, or xe2x80x9ctrepanxe2x80x9d pattern to remove sufficient material to obtain the desired via size. This approach is commonly referred to as spiraling or trepanning with the raw, focused beam. Spiraling, trepanning, and concentric circle processing may generically be referred to as xe2x80x9cnonpunchingxe2x80x9d for convenience.
An alternative approach that is also well known in the art involves passing the TEM00 laser beam with the Gaussian irradiance profile through a circular aperture or mask of a predetermined diameter 12. One or more common refractive optic lenses are then used to project an image of the illuminated aperture onto the work surface. The size of the imaged circular spot depends on both the size of the aperture and optical de-magnification obtained with the refractive imaging lens or lenses. This technique, known as projection imaging or simply imaging, obtains a desired via diameter by adjusting either the aperture size or the optical de-magnification or both, until the size of the imaged spot matches the desired via size. Because the low-intensity xe2x80x9cwingsxe2x80x9d of the Gaussian irradiance profile are masked or clipped by the aperture edges, this imaging technique is, therefore, also called clipped-Gaussian imaging.
When drilling vias with the imaged spot, the laser beam simply dwells at the via site for a number of pulses until sufficient material has been removed. This drilling method, often called xe2x80x9cpunching,xe2x80x9d eliminates the extremely precise and fast in-via movement of the laser spot that is required when trepanning or spiraling with the raw, focused beam. Thus, via drilling with a clipped Gaussian beam reduces the demands placed upon the high-speed beam positioner, since it eliminates the complex small-radius, curved pathways and attendant high accelerations associated with inside-the-via motions. Process development is also simpler with projection imaging because there are fewer process parameters to be optimized.
Clipped Gaussian processing also produces much rounder and more repeatable vias because the inherent variations in laser spot roundness from laser to laser no longer govern the shape of the via, rather the roundness is largely determined by the circularity of the aperture and the quality of the optics used to project its image onto the work surface. Roundness is also secondarily impacted by throughput and the degree to which the wings of the raw Gaussian pulse is clipped. Roundness, or circularity, may be quantified as a ratio of minimum diameter to the maximum diameter typically measured at the top of the via, i.e. R=dmin/dmax. The rounder spots are possible because only the central portion of the Gaussian irradiance profile of the laser beam is permitted to pass through the aperture; hence the low-irradiance outer regions of the Gaussian beam are blocked or clipped by the aperture mask.
A problem with a clipped Gaussian beam is, however, that its center is more brightly illuminated than its edges. This nonuniformity adversely affects the quality of vias created with this beam, particularly blind vias, resulting in vias having rounded bottoms and uneven edges and risking damage to the underlying or neighboring substrate.
A laser system employing the clipped Gaussian technique can be implemented so that varying fractions of the Gaussian beam are blocked by the aperture. If the Gaussian irradiance profile is highly clipped so that only a small portion of the output beam center is allowed to pass through the aperture, then the irradiance profile imaged onto the work surface will be more nearly uniform. This uniformity comes at the expense of rejecting a large fraction of the energy at the aperture mask and hence not delivering the energy to the work surface. Wasting such large portions of beam energy impedes drilling speed.
If, on the other hand, a large fraction of the beam energy is permitted to pass through the aperture, then higher fluence is delivered to the work. However, the difference between the irradiance at the spot center, Ic, and the spot edges, Ie, will be large. The fraction of energy passing through the aperture is commonly known as the transmission level, T. For a Gaussian beam, the following mathematical relationship exists:
T=1xe2x88x92Ie/Ic
For example, if 70% of the beam energy passes through the aperture, then both the irradiance and the fluence at the edge of the imaged spot will be only 30% of the value at the center of the spot. This difference between Ic and Ie causes tradeoffs in the drilling process.
If high laser power is used in order to drill more rapidly, the fluence at the spot center, Fc, can exceed the fluence at which the copper at the via bottom begins to melt and reflow. At the same time, if T is large (and therefore the edge-to-center fluence ratio Fe/Fc within the spot is small), the edges of the imaged spot have low fluence and do not ablate the organic dielectric material rapidly. FIG. 2 is graph of edge fluence versus aperture diameter for clipped Gaussian output under typical via processing parameters. As a result, many pulses are required to clear the dielectric material (such as an epoxy resin) from the edges of the via bottom and thereby obtain the desired diameter at the via bottom. Applying these pulses, however, may damage the center of the via due to the high fluence in that region which melts the bottom copper.
The clipped Gaussian technique, therefore, forces a trade-off between high pulse energy that drills rapidly but damages the center of the via bottom and lower pulse energy that is below the copper reflow threshold fluence but drills slowly and requires many pulses to clear the via edges. Typically, depending on the via size, transmission levels between 30% and 60% offer an acceptable compromise between wasted (blocked) laser energy and the undesirable process phenomena related to non-uniformity of the fluence within the imaged spot. Small vias can be drilled at acceptable speed with lower transmission levels (e.g. 25-30%) and therefore higher uniformity of the imaged spot. However, for many applications, 50% less than T less than 60% is desirable to obtain acceptable speed, and via quality suffers due to bottom copper damage.
A more energy and speed efficient method for drilling vias is therefore desirable.
An object of the present invention is, therefore, to provide a method and/or system that improves the speed or efficiency of via drilling with a Gaussian beam while improving the via quality.
Another object of the invention is to provide such a method or system that employs a UV, diode-pumped (DP), solid-state (SS) laser.
The present invention enhances the projection imaging technique. In one embodiment of the invention, a UV DPSS laser system is equipped with a diffractive optical element (DOE) to shape the raw laser Gaussian irradiance profile into a xe2x80x9ctop hatxe2x80x9d or predominantly substantially uniform irradiance profile. The resulting shaped laser output is then clipped by an aperture or mask to provide an imaged shaped output beam. The imaged shaped output beam has a substantially uniform intensity laser spot from its center to its edge so that high quality vias can be drilled rapidly without risk of bottom damage.
Conventional systems that utilize beam shaping, projection imaging, or diffractive optics employ low brightness non-UV lasers or highly astigmatic and multi-mode Excimer lasers and have been generally used in applications other than materials processing.
In many of these beam shaping applications seeking to obtain near-uniform irradiance, the spatial uniformity is required to make the process work. Without it, the non-uniformity of the fluence at the work surface leads to similar problems with over-processing in the center of the focused or imaged spot and under-processing at its edges. In the present invention, the beam shaping does not enable the via drilling process. Rather, it enhances it by making the process faster and more controllable. The invention therefore provides the ability to enhance the quality, speed, and robustness of the UV laser via drilling process.
Although other types of devices have been used to produce near-uniform or xe2x80x9chomogenizedxe2x80x9d beams with excimer lasers for materials processing, such homogenizers do not work with the highly coherent, near-TEM00 spatial mode of a DPSS high-brightness laser. Further, since unlike the large spots inherent to an excimer laser beam, the TEM00 spatial mode is highly focusable, so the present invention can utilize a much higher percentage of the incident energy.
Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof which proceeds with reference to the accompanying drawings.